February 1932 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
Once again I have given selflessly and freely of my time and talent to make available resources to those seeking certain knowledge regarding vintage radio circuits ;-) This time it is the Radio Service Data Sheet for the Majestic Model 25 - 9-Tube Twin-Detector. The info was published in a 1932 edition of Radio-Craft magazine, one of Hugo Gernsback's many endeavors. Also on the page is a complete list of all such schematics and tuning and repair data posted thus far.
Radio Service Data Sheets
These schematics, tuning instructions, and other data are reproduced from my collection of vintage radio and electronics magazines. As back in the era, similar schematic and service info was available for purchase from sources such as SAMS Photofacts, but these printings were a no-cost bonus for readers.
Three new Majestic receiver models introduced by Grigsby-Grunow Co., Chicago, Ill., have features of special interest to Service Men. These models, the Brentwood (Jacobean Lowboy), Brucewood (grandfather clock pattern), and Cheltenwood (highboy) incorporate "twin power detection," a type of rectification which was used in the Science Museum (London, Eng.) receiver, which was first described in America (at any length) in the April and May, 1931 issues of Radio-Craft; this detector action was still further analyzed and discussed in the November, 1931 issue, page 298. The first reference to the Science Museum set (which incorporated the "'paraphasing" or twin power detection circuit) as a "perfect quality" receiver, was the original title, "A Perfect Quality Demonstration Receiver," in the April issue. It is recommended that Service Men refer to these articles in order to familiarize themselves with the theory and operation of such "two-detector-tube" circuits.
It is inadvisable to use any but Majestic tubes in these receivers, since tube shielding, which is part of the design of those tubes with code numbers terminating in the letter S, is essential to the correct operation of the several circuits.
Operating characteristics for the tube circuits of the Model 25 chassis are as follows: Filament potential, V1 to V8, 2.5 V.; V9, 5 V. Plate potential, V1, V2, V4, 260 V.; V3, 90 V.; V5, V6, 115 V.; V7, V8, 245 V.; V9, 400 V. Plate current, V1, 5 ma.; V2, 1. ,ma.; V3, 3.5 ma.; V4, 5.5 ma.; V5, V6, 14 ma.; V7, V8, 32 ma.; V9, 120 ma. (total). Filament-to-ground potential, V7, V8, 16.5 V. Cathode potential, V1, V4, 3.5 V.; V2, 8 V. Screen-grid potential, V1, V2, V4, 90 V.; V7, V8, 260 V. First filter condenser D.C., 385 V.; second condenser, 330 V. Field coil, 70 V. Measurements to ground at 115 Y .. line potential, and volume control set at maximum.)
The Model 25 chassis employs the efficient superheterodyne circuit designed with a greater number of tuned stages to give the characteristics of uniform high sensitivity, single-channel selectivity (10 kc.), and nearly perfect A.F. response.
The circuit from antenna to speaker is arranged as follows: pre-selector, first-detector, oscillator, I.F. amplifier, second-detector (twin power detection), push-pull pentacle power amplifier stage. Nine tubes are employed in the above mentioned circuits.
The Model 25 superheterodyne circuit differs from the regular superheterodyne circuit employed in the customary receivers, in that the design of the second-detector stage (through the use of twin-detectors) provides an almost perfect fidelity of signal output from the speaker.
The R.F. amplifier, first-detector, and I.F. amplifier are biased by the volume control, and a balance resistor of 160 ohms (contained within the control), which are in the cathode circuit of these tubes. The twin power detectors have in their grid circuit a 1/4-meg. resistor which returns to ground.
Control of volume is obtained in the Model 25 chassis by a 6,500-ohm control which varies the bias of the R.F. amplifier, first-detector, and I. F. amplifier stages.
In cases where low sensitivity is encountered, the first step taken to remedy the condition should be the checking of the R.F. types G-51 and G-51-S tubes. Tubes haying a low amplification factor in any of these positions will seriously affect the sensitivity of the receiver. These procedures should always be taken prior to any attempts at remedy by realignment of the condenser gang.
The power supply system of the Model 25 chassis consists of a power transformer, filter unit, and dynamic speaker field. The filter unit contains two 8-mf. especially designed dry electrolytic condensers, a liberally designed choke, and the speaker field in addition.
In checking the alignment of the Model 25 chassis, the I.F. transformers should not be aligned unless there is definite reason to believe that they are out of adjustment. The setting of these transformers at the factory is more or less permanent, and should not need further adjustment, except in rare cases. In all alignment procedures all output meter must be used,
The procedure in aligning the R.F. and oscillator tuning condensers is as follows: (1), tune in a station in the vicinity of 1,500 kc., or put the output of a local oscillator (if available) into the receiver; (2), align the R.F. stage, and oscillator tuning condenser (the R.F. stage and oscillator aligning condensers are on the gang condenser).
Underview of the model 25 chassis. All parts are labeled for the convenience of Service Men.
To align the oscillator tracking condenser, the steps are as follows: (1), Tune in a local oscillator, set at about 600 kc.; (2), adjust both the tuning control and tracking condenser simultaneously to give maximum signal as noted on an output meter. This will be obtained by "rocking" the tuning control across the resonance point while adjusting the tracking condenser to give maximum output at the point of resonance. (This operation cannot be performed without a local oscillator and output meter.)
The alignment of the RF. oscillator aligning condensers now should be checked in the vicinity of 1.500 kc.
The color code of the power transformer is as follows: Start and finish of primary winding, yellow; start and finish of anode, red; center-tap (anode), black; start and finish of No. 1 heater, black; start and finish of No.2 heater, yellow; start and finish of 5-V. filament, black; center-tap of No. 1 heater, red.
Posted February 25, 2015